docs: def arcball_map_to_sphere(point, center, radius):
      """Return unit sphere coordinates from window coordinates."""
      v0 = (point[0] - center[0]) / radius
      v1 = (center[1] - point[1]) / radius
      n = v0*v0 + v1*v1
      if n > 1.0:
          # position outside of sphere
          n = math.sqrt(n)
          return numpy.array([v0/n, v1/n, 0.0])
      else:
          return numpy.array([v0, v1, math.sqrt(1.0 - n)])